Various types of optical recording media such as CD-R, CD-RW, MO and so forth are widely recognized and spread as external storages for information processing apparatuses such as computers because they can store a large volume of information and can be randomly accessed easily. With an increase in quantity of handled information, there is a demand to increase the recording density.
Among various optical recording media, optical recording media (optical disks) having a recording layer containing an organic dye (also referred to as a dye containing recording layer) such as CD-R, DVD-R, DVD+R and the like are particularly widely used because they are relatively inexpensive and have compatibility with read-only optical disks.
Media such as CD-R representative of optical disks having the dye containing recording layer, for example, are in a laminated structure which has a dye containing recording layer and a reflective layer in order on a transparent disk substrate along with a protective layer for covering the dye containing layer and the protective layer. Recording and reading are performed with a laser beam through the substrate.
DVD-R, which is representative as well, has a laminated structure in which a dye containing recording layer, a reflective layer and a protective layer covering them are formed in this order on a first transparent disk substrate, and a so-called dummy disk, which is a second disk substrate (which may be transparent or opaque) and a reflective layer formed on the second disk substrate is formed on the protective layer through or not through an adhesive layer. Recording and reading are performed with a laser beam from one side of the disk through the first transparent disk substrate. The dummy disk may be of only a transparent or opaque disk substrate, or may be provided with a layer other than the reflective layer. Meanwhile, DVD+R has almost the same structure as DVD-R, description of which will be hereinafter represented by DVD-R.
In order to largely increase the recording capacity of the optical recording medium, two single-sided DVD-Rs as above are bonded together to form a medium having two recording layers, which is known as a double-sided DVD-R (double-sided, dual-layer DVD-R). Recording and reading are performed by irradiating a laser beam onto each of the recording layers from the both sides (that is, the laser beam is irradiated from one side of the medium to perform recording and reading on a recording layer closer to this side, while the laser beam is irradiated from the other side of the medium to perform recording and reading on the other recording layer closer to the other side).
Other than the above, widely known is an optical recording medium (optical disk) having a phase-change recording layer such as CD-RW, DVD-RW or the like. A rewritable optical recording medium having a phase-change recording layer has generally protective layers on and under the recording layer.
With respect to optical recording media having a plurality of recording layers, there is, in these years, a demand for a single-sided incident type optical recording medium (for example, single-sided incident type DVD-R) on which recording and reading can be performed on a plurality of recording layers by irradiating a laser beam from one side so as to avoid an increase in size and complexity of the recording/reading apparatus, and enable continuous reading from the plural recording layers.
To meet the above demand, there has been proposed a single-sided incident type DVD-R of the dual-layer type having two recording layers, for example, as the single-sided incident type optical recording medium having the structure below (refer to Japanese Unexamined Patent Publication No. HEI 11-66622, for example).
For example, a single-sided incident type DVD-R of the dual layer type is formed by laminating, on a first light-transmissible substrate, a first recording layer made from an organic dye on which information can be optically recorded by irradiating a laser beam for recording, a first reflective layer made of a semi-light-transmissible reflective film that can pass through a part of the laser beam for reading, an intermediate layer that can pass through the laser beam for recording and the laser beam for reading, a second recording layer made from an organic dye on which information can be optically recorded by irradiating the laser beam for recording, a second reflective layer reflecting the laser beam for reading, and a second light-transmissible substrate in this order.
In the single-sided incident type optical recording medium having a plurality of recording layers such as a dual-layer, single-sided incident type DVD-R or the like, the optimum recording/reading conditions such as recording pulse strategy (recording strategy, write strategy), recording power, reading power, etc. largely differ depending on each recording layer.
For this, it is desirable that a recording/reading apparatus which performs recording or reading of information on each of the recording layers of a single-sided incident type optical recording medium having a plurality of recording layers can switch the recording/reading conditions such as recording pulse strategy, recording power, reading power, etc., for example, thereby to accurately and surely perform recording or reading of information.
In order to allow the recording/reading apparatus to randomly access to each of the recording layers of the optical recoding medium to record or read information, it is necessary that the recording/reading conditions are instantaneously switched according to the recording layer, in particular.
There is an idea that address information is continuously given to a plurality of recording layers, and the recording/reading apparatus determines which recording layer the relevant layer is on the basis of read address information, for example. However, such manner still has difficulty in instantaneously telling which recording layer the relevant recording layer is.